The present invention relates to a video tape recorder (VTR) of the helical scan type, and particularly to a scan tracking apparatus for a video reproducing head to be employed when a still, slow-motion or quick-motion image is reproduced by the VTR of this kind.
When a specific reproduction such as a still, slow-motion or quick-motion reproduction is performed by a helical scan VTR, the video reproducing head is deflected with a sawtooth waveform to track a video track on a magnetic tape. In this case, it is necessary that the video reproducing head should be controlled to track the center of the video track. Such a technique for tracking the video track is disclosed in the U.S. Pat. No. 4,151,570, whose contents are herein incorporated for reference.
As disclosed in detail in U.S. Pat. No. 4,151,570, a typical helical scan tracking apparatus includes the features which are shown in FIGS. 7, 8, 9 and 10 of the drawings herein. FIG. 7 shows a cylindrical drum 50 with a peripheral side wall 52. The cylindrical drum is rotatable about a central axis 54. A magnetic tape reading head 56 is mounted within the cylindrical drum 50, and it is positioned to be flush with the peripheral side wall 52 of the cylindrical drum. The head 56 is supported on a transducer positional element 58, the positional element in turn being controlled to allow the head 56 to move up and down along a path which is essentially parallel to the rotational axis 54 of the cylindrical drum 50. The cylindrical drum 50 is shown in FIG. 8, with a tape 60 helically wound around it in such a way that the head 56 which is deflectably mounted on the cylindrical drum 50 is capable of tracking video tracks which are recorded on the tape 60. The information is recorded diagonally on the tape 60 as shown in FIG. 10, and due to the helical path that the tape 60 defines along the peripheral side wall 52 of the cylindrical drum 50, and further in view of the fact that the head 56 is allowed to deflect in the longitudinal direction, the head 56 is capable of tracking each discrete recorded video track 66. FIG. 9 shows the head 56 which is mounted on the positional transducer 58 which allows the head to be moveable along the direction shown by arrow 64. The disclosed technique employs a feed back servo system, in which the video reproducing head is vibrated by an output of a dither oscillator, a frequency modulated RF signal derived from the head is subjected to envelope detection, the resultant output is further subjected to synchronous detection with the output of the dither oscillator, and the synchronous detected output is fed back to the video reproducing head through a head drive amplifier. However, this technique requires the dither oscillator and the servo system which means that the construction of the apparatus is complicated, and a complicated operation is required to control the servo system.
In order to eliminate these shortcomings, Sekiguchi et al. proposed a technique on Feb. 28, 1983, in the U.S. Pat. No. 470,667, now U.S. Pat. No. 4,520,410, the disclosures of which are incorporated herein by reference in which a correlation between reproduction outputs extracted from adjacent video tracks is utilized for tracking the video track. In the proposed technique, the reproduction output level from a given video track is compared with a stored level extracted from the preceding video track, and the video head is deflected by a very small amount in a direction corresponding to the result of the comparison.
It is known that, in practice, a video track recorded on the magnetic tape is bent within a certain scope. The bend of the video track is particularly large at edge portions in the video track. The shape and rate of the bend of the video track are fairly similar in VTRs made by the same manufacturer, and therefore accurate tracking can be performed in these recorders using the prior art technique. However, the linearity of the video track, i.e., the shape and rate of the bend at the edge portions in the video track vary between VTRs manufactured by different manufacturers, respectively, and thus accurate tracking is not achieved when a tape recorded by a VTR of one manufacturer is played on a VTR of another manufacturer. In the worst case, this results in the appearance of noise in the reproduced picture.